Performance improvements in commercial vehicles (CVs) that serve to reduce driver fatigue as well as increase comfort and operability are important in the industry’s efforts to recruit and retain more drivers. Consequently, the use of mechanical automatic transmissions (T/Ms) and multi-stage designs in CVs is becoming more widespread, which tends to increase weight. Reducing fuel consumption not only calls for making vehicles smaller and lighter, but also for improving allowable torque and enhancing the strength of components. Achieving a balance between these technologies is currently a significant challenge, according to experts from Hino Motors.
In T/Ms with a synchronized mesh mechanism, there is a strong need to increase the wear resistance of sleeves and dog gears. Structurally, these parts are subject to impact loads at high surface pressures and thus require high wear resistance. Shorter shift times are also expected to cause higher surface pressures.
Repeated impacts and sliding and the complex motion of sleeves and dog gears mean that their wear mechanism is poorly understood. Furthermore, the existing wear testing apparatus is unable to evaluate such complex wear patterns. Development at Hino Motors thus followed these steps: elucidating the wear mechanism, designing the material, planning and validating the evaluation method, and evaluating the test material.
Elucidating the wear mechanism
Air controls the movement of the sleeve in T/Ms with a synchronized mesh mechanism. The system works as follows: shifting pushes the sleeve toward the dog gear, which then pushes the synchronizer ring onto the cone of the dog gear, where the friction between the inside of the ring and cone synchronizes the rotations of the sleeve and dog gear. Once synchronized, the sleeve moves forward past the synchronizer ring. The apexes of the splines on the inner periphery of the sleeve and the chamfered section on the outer periphery of the dog gear have pointed portions (chamfers). The sleeve slides the dog gear into the engagement position and fully engages with the dog gear when their respective chamfers come into contact.
In this process, impact and sliding with the dog gear creates a load exceeding 4 kN (900 lb) on the sleeve. In addition, extreme surface pressures are predicted because the contact between the sleeve and dog gear chamfers during shifting is not uniform and involves significant changes in the size of the contact area.
The researchers reviewed the results of macro- and micro-observation of the sleeve chamfers after the durability tests. Macro-observation of the surface reveals significant abrasion and curving of the apexes of the chamfers, while micro-observation brings fine peeling and plastic flow to light. In the cross-section, macro-observation reveals wear significant enough to virtually eliminate the hardened layer, while micro-observation shows a white layer structure typical of adhesive wear on the outermost surface layer, as well as a remarkable degree of plastic flow and cracking.
Based on these observations, wear on sleeve and dog gear chamfers can be identified as a combined pattern of peeling resulting from the propagation of fine cracks caused by the large loads generated by impact and sliding, as well as of adhesive wear, given the presence of a white layer and plastic flow.
Designing the material
From the standpoint of preventing gear slippage, using a smaller chamfer angle and reducing the load via its shape is problematic, so a material-based solution was sought. Sleeves and dog gears currently use a carburized, tempered low alloy steel (the “base” material for this study) with enhanced surface hardness and hardened layer depth.
Based on the wear pattern, suppressing fine cracks and adhesion should be effective in reducing wear due to impact and sliding. Ways to suppress crack formation and propagation include strengthening grain boundaries and improving toughness. Grain boundary strengthening can be achieved by suppressing the segregation of P (phosphorus) at grain boundaries and the precipitation of cementite. Since adding Mo (molybdenum) leads to Mo carbide precipitation centered on the transition within the prior austenite grains, grain boundary breakdown is suppressed by the finely dispersed precipitation of the precipitates within the prior austenite grains.
Next, B (boron) is an element that can improve internal toughness while simultaneously strengthening prior austenite grain boundaries by segregating them and suppressing their P precipitation through accelerated diffusion. Further, the addition of Ti-Nb (titanium niobium) and Al-N (aluminum nitride) is expected to improve toughness since it suppresses grain growth during carburizing, which enables the formation of fine crystal grains.
Adhesion can be suppressed through methods such as improving surface hardness, including temper softening resistance. However, due to the impact input on sleeves and dog gears, greatly increasing surface hardness leads to concerns over chipping due to reduced toughness, so surface hardness was kept at a level equivalent to that of the base.
Elements that can improve temper softening resistance include V, W, Mo, and Si (vanadium, tungsten, molybdenum, and silicon, respectively). Since instances of continuous impact or sliding are uncommon in actual parts, little heat is expected to be generated. For the Hino Motors research, Si, which can improve softening resistance at relatively low temperatures in the vicinity of 300°C (572°F), was chosen. In phase 3, carbide dispersion is an issue. Since the addition of a carbide-producing element such as W makes the entire raw material hard and reduces machinability, heat treatment was used to apply carbide dispersion.
Alloys with added Mo, B, Ti-Nb, Al-N, and Si were made with the goal of strengthening grain boundaries and improving toughness and temper softening resistance. (View table in images above to see the chemical composition of materials.) The carbide dispersion carburizing heat treatment, which makes it possible to confirm the effects of the carbide, was also tested.
Evaluation results and observations
The evaluation of sleeve and dog gear wear due to impact and sliding is performed through T/M assembly durability bench tests, but this presents problems such as long evaluation times. Therefore, a testing apparatus was built that simulates the gearshifting action of sleeves and dog gears, and can reproduce the wear patterns found in actual parts.
The variation in wear volume for 100,000 test cycles was determined. Compared to the base, significant reductions in wear were observed: 21% for S1, 58% for S2, 54% for S3, and 78% for B+SC.
The greater wear in S1, which contained more added Si and Mo than in S2 and S3, as well as the small amount of difference between the latter two, indicate that the addition of B, Ti-Nb provides better impact wear resistance due to grain boundary strengthening and better toughness improvement than the addition of Mo or Si.
The factors contributing to wear reduction in S3, which is expected to reduce material cost since it contains no Mo, were analyzed. SEM photographs of the early stage of wear for cross-sections of the base and S3 after 5000 test cycles reveal peeling and a large number of fine cracks on the base. By contrast, S3 has little peeling and few cracks, suggesting that improved toughness suppressed cracking. Furthermore, crack propagation on S3 was shallower than on the base.
Toughness was evaluated by substituting Charpy impact values. To evaluate the carburized layer, the specimens were cut to a thickness of 2 mm (0.08 in), a special specimen hardened through its entire thickness was prepared, and impact values were compared. S3 had impact values twice as high as the base.
The findings suggest that one factor in wear reduction is the suppression of cracking and lessening of peeling resulting from the improved toughness and strengthened grain boundaries obtained by adding B and Ti-Nb. Another factor is the facilitating of deformation texture formation due to the addition of Ti-Nb, which leads to shallower crack propagation directions and reduced peeling.
Durability bench tests were performed on a T/M assembly using sleeve and dog gear prototypes based on S3. After a number of test cycles equivalent to vehicle service life, S3 exhibited a 45% reduction in wear depth compared to the base.
This article is based on SAE International technical paper 2015-01-0517 by Masaaki Kawahara, Noriaki Katori, and Tatsuya Koyama of Hino Motors Ltd.